The present invention relates to optical interferometry and an interferometer utilizing the interference phenomenon of laser beams. More particularly, it relates to a wavelength scanning interferometry and an interferometer with a laser diode that is well-suited for application to, e.g., the inspection of optical elements of which a high precision is required for reading an interference pattern.
In a prior-art optical interferometer, the wavelength of a light source is fixed, and a method such as providing a beam with a frequency shifter or modulating the phase of light by means of a piezoelectric device is employed for raising the reading precision of an interference pattern. However, both the devices for providing these methods are expensive. Other disadvantages are that a large number of components of these systems must be adjusted and that a driving power source of high voltage is required.
More specifically, in a prior art optical interferometer based on the so-called heterodyne interference method, the wavelength of a light source is fixed. In order to measure the phase of the wavefront between interference fringes, a method is employed in which the wavelength of a beam is shifted by a frequency modulator or in which a piezoelectric device is mounted on a mirror so as to perform phase modulation. Both of the methods, however are disadvantageous in that the modulation devices are expensive and require high voltages. The latter method involves complicated handling for the corrections of a nonlinearity and a hysteresis, etc.